For much of the past six months, if you asked a Democrat about Massachusetts Republican Senator Scott Brown, you’d hear some mix of exasperation and amazement. In a state that went for Barack Obama by 26 points in 2008, Brown, who won the late Ted Kennedy’s seat in a 2010 special election, has been more than holding his own against liberal icon Elizabeth Warren. Brown’s combination of homespun appeal and a knack for playing the middle on the issues is “sadly masterful,” one Democrat told me at the Democratic National Convention in Charlotte. And it has more than a few Massachusetts Dems, who outnumber registered Republicans in the state by a 3 to 1 margin, carefully weighing their options.

Until this week, in fact, the most important uncommitted Democrat in the country was probably Boston Mayor Tom Menino, the kind of big-city, big-personality, machine politician who seems to have stepped out of another age; a majority of the residents of Boston say they’ve personally met Menino and an even high number say they like him. For months, he praised both Brown and Warren — once even declaring the Republican Senator unbeatable — but declined to endorse either of them. Warren’s campaign, anxious about getting conservative urban Democrats to vote for a Harvard professor over a truck-driving National Guardsman from Wrentham, didn’t relish facing Election Day without Menino’s turnout machine on its side.

Now, as it turns out, it probably won’t have to. Menino’s endorsement is expected to come Friday, according to the Dorchester Reporter, and it’s just the latest in a series of auspicious signs for Warren and Democratic Senate candidates like her. As volatile polling clouds the presidential race and pundits diagnose the Romney campaign’s alleged ills, it’s actually the GOP’s effort to take the Senate, not the White House, that’s in grave condition.

It wasn’t supposed to be that way; 2012 was widely viewed as a very favorable year for the GOP: Democrats are defending 23 of the 33 contested seats and will lose their majority if they lose a net four of them. But over the past month, something has gone seriously wrong. Nate Silver’s forecasting model lurched from giving Democrats just a 39% chance of holding the Senate in mid-August to a 79% chance this week.

The shift is evident across the country. In Florida, incumbent Democrat Bill Nelson has opened up a considerable lead on Republican challenger Connie Mack, with four recent major polls in the state finding Nelson 7 to 14 points ahead. The race had been a tie in July. Former Virginia governor and DNC chair Tim Kaine is leading George Allen by about 4 points, with recent polling showing him ahead by as much as 8. This, after months of deadlock. In Ohio, Democratic Senator Sherrod Brown’s once narrow lead has widened to about 7 points, and in Wisconsin, Representative Tammy Baldwin has just taken a thin lead in the polls after months of trailing Republican former governor Tommy Thompson.

Each race is different, and these close contests are far from over. But Brown’s case in Massachusetts is instructive.

Throughout the spring and summer, Massachusetts played host to one of the most closely watched Senate races in history. Outside spending groups showed up in force before the candidates signed a cease-fire agreement. When Obama borrowed and botched Warren’s words by saying “You didn’t build that,” her line about businesses owing the government some credit for their success became a national flashpoint. Controversy erupted over Warren’s claim of Native American heritage during her teaching days. Through it all, the the polls barely budged. Brown and Warren were tied, and nothing seemed to change that. Until early September.

Four recent polls suddenly showed Warren grinding out a small lead. Part of that change may have to do with Warren herself; voters are getting to know the political novice as advertising picks up and she comes off a plum speaking slot at the DNC. A Suffolk University survey found her net approval rating increasing nine points, to +19, from May to September.

But more important, Brown is losing crossover appeal. The number of Democrats who said they were backing the Senator decreased by more than a fifth since May, even as Brown’s robust +31 popularity in the state hardly moved. And if you listen to the candidate himself, he seems worried that his interparty appeal is being compromised by the national Republican Party.

Brown was quick to disavow Todd Akin’s comments on rape and even quicker to distance himself from Mitt Romney’s videotaped remarks disparaging the 47% of Americans who don’t pay federal income taxes. “I’m Scott Brown, he’s Mitt Romney. We disagree on a whole host of things,” Brown told TIME on Wednesday. “I lived in 17 houses by the time I was 18. My mom got public assistance for a short period of time. I don’t think anybody is on public assistance because they want to be.”

But it isn’t just the case of one moderate Senator in a liberally inclined state. Senator Dean Heller of Nevada, locked in a tight race with Representative Shelley Berkley, similarly disavowed Romney’s worldview espoused in the video posted by Mother Jones. “I have a very different view of the world, having grown up, as I said, with a father who was an auto mechanic and a mother who was a school cook and five brothers and sisters,” Heller said. “I believe in a safety net. I believe that’s one of the responsibilities of the federal government.”

Wisconsin’s Thompson was more blunt in a local TV interview. “You know, whether you’re a Democrat or Republican, if your standard bearer for the presidency is not doing well, it’s going to reflect on the down ballot,” he said.

I have no idea what pushed Menino over the edge for Warren. Maybe it was always in the cards. But Republicans like Scott Brown need Democratic supporters to win. A juiced conservative base may (or may not) help Romney nationally, but ideological warfare appears to be damaging Republican prospects down ballot. And regardless of who wins the White House, control of the Senate will help determine what the next President is able to accomplish with his term.